1. Field of the Invention
The present invention relates to a dielectric ceramic composition to be used in the frequency region of microwaves.
2. Description of the Prior Art
As the communications method utilizing electromagnetic waves in the frequency region of microwaves has been progressed in past years, for example, in the automobile telephones, portable phones or satellite communications, etc., miniaturization of the apparatus is strongly required. That is, miniaturization of each components constituting the apparatus is demanded. A dielectric material is incorporated in a filter or an oscillator of the apparatus as a dielectric resonator. The size of the dielectric resonator is inversely proportional to a square root of the dielectric constant of the dielectric material when same resonant mode is used, and therefore a dielectric material with a high dielectric constant becomes necessary in order to a compact size dielectric resonator is realized. Besides, the loss of the dielectric material against microwaves should be small, i.e., the dielectric material should have a high Q factor, with a small temperature change of the resonant frequency so that the dielectric material is applied to an actual resonator.
Many ceramic compositions have ever been developed for use in the dielectric resonator. In these compositions BAO-TiO02-SM203 type has been disclosed as a ceramic compositions of a particularly high dielectric constant in U.S. Pat. No. 4,330,631 dated May 18, 1982. This type of ceramic compositions assumes approximately 80 dielectric constant, and as high as about 3000 Q factor at 2-4 GHz, and a small temperature coefficient of the resonant frequency. Also, Ba-PbO-Ti02-Nd203 type has been reported in the Journal of American Ceramic Society, Vol. 67 (1984), p. 278-281 as a ceramic composition having a dielectric constant of not smaller than 90.
Not only the above-described types of ceramic compositions, but the conventional ceramic compositions of a high dielectric constant for microwave use have a high sintering temperature, namely, 1300.degree. C. or so and are of mixed phase. Therefore, the microwave characteristics of the mass-produced ceramic composition are greatly varied and, manufacturing costs high.